1. Field of the Invention
This invention relates to a rear conversion lens mountable to the image side of a photographic lens to increase the focal length thereof.
2. Description of the Prior Art
A rear conversion lens is very convenient in that it is simply mountable to the image side of a conventional lens to thereby enable the latter lens to be used as a lens of greater focal length. Also, if the rear conversion lens is small in size and light in weight, it increases its attractiveness as an interchangeable lens and becomes easier to operate.
A conventional rear conversion lens of such type has employed a negative lens system of strong refractive power which has tended to increase the Petzval sum in the negative sense, and also to increase astigmatism as well as the other remaining aberrations. Thus, however high may be the performance of the master or conventional photographic lens to which the conversion lens is attached, the performance thereof has tended to be greatly diminished when a rear conversion lens is mounted thereto. Moreover, if the master lens is of a large aperture ratio, the spherical aberration and the other various aberrations must be well corrected to permit a sufficient quantity of light to pass through the rear conversion lens, and if it is desired to make the rear conversion lens itself of a small size, the various aberrations become more pronounced, and this has made it difficult to design a compact lens which is usable with a master lens of large aperture ratio.